Typical reaction products of epoxy resins with cross-linking curing agents include secondary alcoholic hydroxyl groups which were present as such in the starting epoxies or have been produced by ring opening reactions of oxirane groups with hydroxyls (or other active hydrogen source groups). The alcoholic hydroxyls in the cured epoxies attract and provide binding sites for environmental water. The water acts as a plasticizer and thereby lowers the tensile and flexural strengths and the modulus of the cured resin. It also increases the dielectric constant of the resin.
It might be expected that formation of hydroxyls would be avoidable by using anhydrides to cure epoxies. In fact, however, because active hydrogen compounds must be used with anhydrides as promoters and because it has been found empirically necessary to use less than stoichiometric amounts of anhydrides, the presence of hydroxyls (alcoholic and/or carboxylic) in the cured epoxides is not so avoided.
Accordingly, other epoxy curing reactions which might not result in hydroxyl group formation were sought. A candidate such reaction would appear to be oxirane/ester or oxirane/carbonate reaction.
In 1974, it was reported (L.I. Komarova et al., Vysokomol. Soedin., B16, 718, 1974) that the ready reaction of any of several different types of diepoxides with a variety of polyesters resulted in curing (hardening).
In 1978, it was further reported** that the previously noted curing was due to reaction of ester groups with oxirane rings. Utilization of epoxy oligomers as hardeners for polyesters was suggested. The following type-reaction--which does not produce hydroxyls--was disclosed. ##STR1## FNT **Komarova et al., Journal of. Polymer Science, Polymer Chem. Ed., Vol 16, pp. 1643-1657 (1978): John Wiley & Sons, Inc.
Also reported ***--in 1979--were base-catalyzed adductions of aryl esters (phenyl acetate, phenyl benzoate, S-phenyl thiobenzoate) and diphenyl carbonate with monofunctional epoxides (ethylene and propylene oxide, styrene oxide and phenyl glycidyl ether). FNT ***K. Funahashi, Bulletin of the Chem. Soc. of Japan, Vol. 52(5), pp. 1488-1492 (1979)
A recent paper **** discloses the formation of an "Epoxide-Polycarbonate Copolymer Network" when a phenol-terminated bis-phenol A-type polycarbonate oligomer is reacted with a stoichiometric amount of an oligomeric digycidyl ether of bisphenol A advancement product with bisphenol A. The network has the structure of a "phenoxy resin" cross-linked through carbonate groups. The reaction is catalyzed by quaternary ammonium salts, tertiary amines and alkoxides (the latter two types of compounds also catalyzing homopolymerization of epoxides). The density of cross-linking in the network can be controlled by adjusting the number ratio of carbonate to oxirane groups, each carbonate being able to react out two oxiranes. The copolymer product is depicted (in a partial structure) as containing secondary alcoholic hydroxyls--presumably present in the oligomeric epoxide used. FNT ****Yu and Bell, Journal of Polymer Science: Part A. Polymer Chem. Vol. 266, pp. 247-254 (1988) John Wiley & Sons.
According to U.S. Pat. No. 4,782,124 (1988), it is possible to modify epoxides which contain alcoholic hydroxyls by reacting the hydroxyls with aromatic carbonates, using a transesterification catalyst (such as, for example, ethyl triphenylphosphonium acetate--as such or as "A-1", the complex with a molecule of acetic acid--ethyltriphenyl phosphonium phosphate or tetrabutyl phosphonium phosphate).
Thus, although the prior art does not itself suggest doing so, it would appear that even epoxides containing active hydrogen-containing groups could be converted (as by transesterification) to hydroxyl-free epoxides suitable for preparation of hydroxyl-free copolymers of the type disclosed by Yu and Bell.
However, the prior art known of by the present inventor does not provide much guidance as to whether or not ester or carbonate cured epoxies would have physical properties making them suitable for one or more commercial applications. It is known that blends of epoxides and carbonates are substantially tougher than the epoxides alone and the Yu and Bell paper shows Tg's (glass transition temperatures) of up to 123.degree. C. for copolymer networks (of the disclosed type) at several reactant values. (The highest Tg given for the "linear phenoxy" resins per se--when self-converted by heating, is about 60.degree. C.) However, no indication as to melt processability of the copolymers or of other cured product properties is provided.